Field of the Invention
The invention relates to a high-temperature fuel cell having at least one electrically insulating covering, as well as to a method for producing a high-temperature fuel cell.
It is known that live metallic components connected to one another in high-temperature fuel cells must be partially electrically insulated from one another. It is particularly difficult to satisfy that requirement in the case of a high operating temperature, for example an operating temperature of more than 800.degree. C. Frequently, an integral material connection between the metallic components can be produced only with a glass-like substance, for example with glass or glass solder, due to specific boundary conditions. However, such a glass-like substance is normally relatively highly electrically conductive at a high operating temperature and is consequently a poor electrical insulator.
Furthermore, those glass-like substances have the characteristic of decomposing when an electrical voltage is applied to them. That decomposition may take place even at a voltage of considerably less than 1 volt.
A report entitled "Materials for Solid-Oxide High-Temperature Fuel Cells" by W. Drenckhahn et al., in Siemens Power Journal, Issue 4, Year 94, pages 36 to 38, in particular, discloses the application of an electrical potential of, for example, 0.7 volts operating voltage to 1.3 volts quiescent voltage between two bipolar plates in high-temperature fuel cells. Even at those electrical voltages, decomposition reactions occur between the bipolar plates as a result of electrolysis, if a glass-like substance is used, that is to say ion migration takes place from one electrode side to the other. In the process, the intrinsic strength of the glass-like substance is adversely affected even after a short time, as a result of which the probability of a malfunction occurring is in turn considerably increased.
In order to avoid that disadvantage, the bipolar plate is provided with an additional single-layer covering of stabilized ZrO.sub.2 which, depending on the requirement or the structure, has a thickness of between, for example, 30 .mu.m and 150 .mu.m. Optionally, for better adhesion of the covering on the bipolar plate, a so-called adhesion promoter composed of metal or ceramic is applied to the bipolar plate to improve the adhesion of the covering on the bipolar plate. The ZrO.sub.2 material that is used in general is partially or fully stabilized using known technical stabilization components such as Y.sub.2 O.sub.3, CaO, MgO, Al.sub.2 O.sub.3 or CeO.sub.2.
That single-layer covering has inadequate electrical insulation which is required especially if a glass-like substance is used to join the metallic components together, since the glass-like substance is a poor electrical insulator.
U.S. Pat. No. 5,338,577 discloses a method for coating a metallic substrate with ceramic through the use of flame spraying, wherein an intermediate covering of steel is sprayed on initially, followed by a covering of yttrium-stabilized ZrO.sub.2 and finally an Al.sub.2 O.sub.3 covering.
Furthermore, Japanese Published, Non-prosecuted Patent Application No. 06-144 971 discloses a covering body which includes a ceramic substrate, adhesion promoter coverings of steel, a ceramic covering, and possibly an outer covering to close the pores, wherein the adhesion promoter coverings and the ceramic covering are produced through the use of flame spraying, and the outer covering is produced through the use of a sol-gel technique.
In addition, US Statutory Invention Registration H 12 60 discloses a method for producing covering bodies for high-temperature fuel cells through the use of a plasma spray process. However, in the case of that process, only a single electrically insulating covering, composed of yttrium-stabilized ZrO.sub.2, is sprayed on.